392 CHEMICAL ENGINEERING size and type of apparatus. The drying may be done at atmospheric pressure, and in this case it is essential to remove the vapors by suction, as otherwise the condensation of these vapors will interfere with the drying process. The application of vacuum has many advantages, as it will lower the boiling temperature of the solvent, and reduce the steam consumption. However, vacuum dryers are expensive to install and operate, and can only be used for valuable products where a low temperature is essential. FIG. 32.—Trough dryer. Various Types of Steam Dryers.—A simple apparatus is a jacketed shallow pan which is shown in Fig. 31. A number of such pans are frequently placed in one chamber, the outlet of which is connected to an exhauster or chimney. The material rests directly on the pan surface or is placed in metallic trays which can easily be handled. Figure 32 describes a trough dryer which may be used in batches or continuously. Here the material is agitated by paddles or screws and the capacity per square foot is therefore much higher than in tray dryers. Steam consumption is high, i.e., from 1.6 to 2.0 Ib. per pound of evaporation. FIG. 33.—Rotary dryer. Similar to the trough dryer is the so-called rotary dryer specified in Fig. 33. This may be used for either liquids or solids, and the capacity and efficiency are good. Both the outside jacket and the interior drum are heated by either steam, hot water or oil, and the heating surface is kept clean by the scrapers attached to the interior revolving drum. From 1.3 to 1.5 Ib. of steam are required to evaporate 1 Ib. of water. Figure 34 shows a tubular dryer which can be arranged so that the steam or heating medium is inside the tubes, and the material on the outside, or vice versa. In either case it can only handle material of granular nature which wil] not stick to the surface. This type of apparatus gives generally a large capacity, as the heating surface can be arranged in compact form. The steam consumption is low, that is about 1.3 Ib. per pound of water evaporated. A continuous plate dryer is specified in Fig. 35.